(1) Field of the Invention
The present invention relates to the field of assisting piloting a rotorcraft, and in particular for stages of flight close to hovering or at a low advance speed during an approach. The present invention relates to a method and to a device for detecting that a rotorcraft, in particular a rotorcraft of the helicopter type, is approaching the vortex domain, and for signaling that detection.
More particularly, this method and device for detecting and signaling are intended to warn the pilot of a rotorcraft that it is close to, or even entering, a region of its flight envelope that is generally referred to by the person skilled in the art as the “vortex state”.
(2) Description of Related Art
A rotorcraft, also known as a “rotary-wing aircraft”, has at least one main rotor with a substantially vertical axis providing the aircraft at least with lift. The main rotor has blades driven to move in rotation. In the particular circumstance of a helicopter, the main rotor, as driven by at least one engine, provides both lift and propulsion. As a general rule, a helicopter also has an auxiliary anti-torque rotor for controlling the yaw movement of the rotorcraft.
Consequently, at least one engine delivers mechanical power to the main and auxiliary rotors, and also to other equipment via a main power transmission gearbox (MGB).
Under such conditions and ignoring particular stages of takeoff and landing and of turning, a rotorcraft performs three kinds of flight in principle:
vertical flight, upwards or downwards;
hovering flight, with the rotorcraft remaining stationary; and
flight in horizontal translation.
The invention relates to downward or “descending” flight, mainly during stages of approach for the purpose of landing, and also while approaching hovering flight.
Specifically, during descending flight, the flow of air generated by the main rotor differs depending on whether descent is fast, moderate, or slow.
Fast and moderate descending flight generally comprises regimes that are not motor-driven. The necessary power is provided by the stream of air and a freewheel interposed in the mechanical power transmission system allows the main rotor to turn freely.
In contrast, slow descending flight is a motor-driven regime, with the pilot causing the rotorcraft to descend and controlling that descent by varying the collective pitch of the blades of the main rotor.
The invention relates more specifically to slow descending flight of a rotorcraft, e.g. from a hovering position, this descent possibly taking place purely vertically or else along with a steeply sloping flight path, i.e. with a small horizontal speed component referred to herein as the “instantaneous proper speed” VP of the rotorcraft. This instantaneous proper speed VP remains in a relatively small range of values and is associated with an instantaneous vertical speed.
During a slow descending flight, a wake forms from the bottom portion of the main rotor, thereby obliging the bottom central air streams to be deflected downwards and the top central air streams to create a zone of turbulence towards the periphery of the blades. Aerodynamic flow is then disturbed and peripheral vortices run the risk of developing and completely isolating the plane of the main rotor. This dangerous phenomenon, referred to as the “vortex state” leads to a general loss of lift and controllability for the rotorcraft.
Under such conditions, when a rotorcraft in hovering flight begins a vertical descent, the reversal of the direction of speeds runs the risk of preventing the air stream from passing through the main rotor whether upwards or downwards. The blades are then working in their own wash and the air forms a turbulent ring around the main rotor. This vortex state gives rise to vibration that is dangerous for all rotorcraft and runs the risk of leading to a loss of control.
As a general rule, the turbulent ring develops at a vertical speed that is equal to about half the mean induced speed during hovering flight outside the ground effect zone and with an advance speed that is small or substantially zero. A large portion of the main rotor is then in a stall zone, with the various elements of the blade then operating with an angle of incidence that is relatively large. When the rotorcraft is moving in translation, the wash from the main rotor is deflected rearwards, and as a result the vortex state does not occur.
The vortex state is to be feared since the rotorcraft becomes isolated from the mass of air in which it is flying. The vertical speed indicator can then reach large undesired values. It takes a considerable amount of time to leave a vortex state. The three main ways of entering into a vortex state are as follows:
hovering flight with uncontrolled drift of vertical speed;
approach stages for the purpose of landing where the vertical speed for potentially entering into a vortex state is displayed with a uncontrolled reduction in the advance speed and/or with the collective pitch of the blades of the main rotor being raised too late, and thus with flight power being raised too late; and
uncontrolled stages of slowing down for the purpose of performing hovering flight, e.g. in order to perform winching or as a result of poor weather conditions with the collective pitch of the blades of the main rotor being raised too late.
In most circumstances, a vortex state is entered into at low altitude, and as a result the vertical speed of the rotorcraft and also the time required to leave this vortex state generally lead to the rotorcraft crashing. A vortex state is in some ways equivalent to stalling as observed with airplanes.
One known situation for entering the vortex state is during “quasi-vertical descent”. The person skilled in the art qualifies such a vortex domain as “static”. The attitude of the plane of the main rotor and the longitudinal attitude of the rotorcraft are both considered as being substantially zero.
Although this situation does indeed occur, it is not the situation that occurs the most frequently, since it does not correspond to a conventional way of using a rotorcraft.
The person skilled in the art also refers to a “dynamic” vortex domain, corresponding to the presence of the advance speed of the rotorcraft decelerating strongly. This dynamic vortex domain is characterized by an attitude angle θ for the plane of the main rotor relative to a plane normal to the gravity direction, i.e. associated with a terrestrial reference frame, that is of the order of 20°, or even more, for example. This angle of inclination of the plane of the main rotor is the result of a pilot of the rotorcraft acting on the cyclic stick controlling the longitudinal cyclic pitch of the blades of the main rotor, and it is accompanied by a variation in the longitudinal attitude θ of the rotorcraft, which substantially follows the angle of inclination of the plane of the main rotor.
This dynamic vortex domain is just as dangerous as the static vortex domain and, by way of example, it can occur when landing with a vertical descent speed and a large amount of horizontal deceleration, which situation can sometimes be made worse by a light tail wind.
As a result, a static vortex domain is defined essentially by a single vortex state such that the attitude of the plane of the main rotor is substantially zero. In contrast, there may be a plurality of dynamic vortex states. Each dynamic vortex state corresponds to a given attitude of the plane of the main rotor.
A vortex domain is thus dangerous, but it can be abandoned by the pilot initiating movement in translation by modifying the cyclic pitch of the blades of the main rotor. The technique recommended in some works for escaping the vortex domain, whereby the downward vertical speed of the rotorcraft is increased by reducing the collective pitch of the blades of the main rotor so that the main rotor escapes from its own wake, is indeed possible, however it is not realistic in operational situations where the height of the rotorcraft above the ground is low when the vortex state appears.
The technique that is recommended in operation for escaping from the vortex state is thus to increase the longitudinal cyclic pitch in a forward direction.
In order to anticipate the risk of the rotorcraft entering into a vortex domain, warning methods and systems exist for the purpose of alerting the pilot of a rotorcraft that flight is close to a vortex domain or is indeed already in a vortex domain.
For example, Document EP 1 950 718 describes such a system and such a method, in which a warning is triggered when firstly the tail wind speed to which the rotorcraft is subjected is greater than a first threshold determined as a function of the height of the rotorcraft relative to the ground, and secondly the descent speed of the rotorcraft is greater than a predefined second threshold. In addition, inhibition conditions serve to avoid issuing a warning when the rotorcraft is flying with a rate of change of heading or a rate of change of advance speed that is large. In contrast, that system and that method function only while the rotorcraft is flying with a tail wind.
Also known is Document FR 2 921 635, which describes a method and a device serving to detect that a rotorcraft is entering a vortex domain or indeed, in predictive manner, that the rotorcraft is approaching such a vortex domain. Such predictive detection is performed as a function of a predictive proper speed and a predictive vertical speed of the rotorcraft, which are determined in real time and possibly corrected.
Furthermore, Document EP 2 513 732 describes a system and a method serving to detect whether an aircraft is close to or indeed already in an aerodynamic stall situation, followed by engaging an automatic procedure for avoiding entering into a dangerous situation or else for escaping therefrom. Such a situation is detected by comparing a vertical speed error between the current vertical speed and the setpoint vertical speed with an error threshold and verifying its sign. The vertical acceleration and the advance speed of the aircraft can also be compared with respective thresholds in order to verify whether the aircraft has escaped from the stall situation.
Furthermore, Document US 2011/0295568 describes a method of determining changes in the geometry of the vortex state caused by the blades of the main rotor of the aircraft depending on the vertical induced speed of the main rotor and on the lift distribution of the blades of the main rotor.
Also known is Document FR 2 978 586, which describes a method and a device for assisting piloting for use in a hybrid aircraft having a main rotor and at least one propulsive propeller. That method makes it possible to define a minimum flight path angle that the aircraft can follow in descent as a function of a thrust margin for each propulsive propeller. That minimum angle may be used in particular in order to avoid the hybrid aircraft entering into a vortex domain.
Finally, Document WO 2004/101358 describes a flight control system for a rotorcraft that makes it possible to avoid a vortex state appearing. That flight control system acts on the collective and/or cyclic pitch of the blades of each main rotor, e.g. in oscillatory manner, so as to generate disturbances on its blades and thus avoid the appearance of the vortex state.